Java Applications

This is David Schwartz's attempt to give a concise explanation of the command-line enviroment and other related things. The document is based on the handout supplied to David Gries's CS100J students in Spring 2003, Sun's on-line tutorial and downloading instructions, and numerous postings from CS100/CS211 staff. For more information, see Sun's tutorial. Some sections are still under concstruction, given the upcoming changes in Java 1.5.

• 1. Java Programs
• 2. Command-Line Environments
• 3. Command-Line Environments Arguments
• 4. Development Environments
• 5. Stand-Alone Java Applications (JAR Files)
• 6. Javadoc

1. Java Programs

1.1 Applications and Applets

You can write Java programs in two general ways:

• Java applet: Program that run a browser. See http://java.sun.com/docs/books/tutorial/applet/index.html.
• Java application: Program that runs "on its own" (not from a browser) by executing the main method of a given class. A Java application is typically composed of many classes.

1.2 Main Class

To be an application, one Java class must contain a main method. The main method typically has this header:

public static void main(String[] args)

1.3 Files and Classes

This section describes general rules for where you place your code:

• All code (except for import statements) must be written inside a class.
• One class must be public and contain a main method. The main method header must declare main as public static void:
  • public: allow an outside source/user to call the method.
  • void: the main method returns nothing.
  • static: the outside user/source doesn't create an object of the Main Class. However, the code inside the class can create an object of that class. For example,

    	  public class Thing {
                 public static void main(String[] args) {
    	        Thing t = new Thing(); 
                    /* code that uses t */ 
                 } 
              }

• Each class should be in a separate file called Classname.java. You can break this rule, but we strongly advise against doing so!
• Each file should contain one public class. All other classes should not have public in their headers. You can actually break this rule if you a database-driven IDE, like CodeWarrior, but we strongly advise against doing so!

2. Command-Line Environments

2.1 Operating Systems and Window Environments

The advice herein is really just a snippet of Sun's instructions at http://java.sun.com/docs/books/tutorial/getStarted/cupojava/index.html:

• Command windows: Assuming you download a copy of the JDK, you can compile and run your programs from a command window. A command window is special kind of window in which you enter text-based commands to your operating system to perform a variety of tasks. Before we had windowing environments, text-based screens were all we had. Talk to some older computer users, and they will regale you will tales of teletype and punchcards....
• Command line: The area in a window in which you type commands. You press Enter or Return after typing your commands.
• Command prompt: A special collection of characters that indicate where you should type your command line.

2.1.1 Using Microsoft Windows

Accessing command windows:

• From Start menu, select Run. In the window that pops up, enter cmd.
• Try also Start→All Programs→Accessories→Command Prompt.
• For more information, see Sun's tutorial.

Using MSDOS: for a full set of DOS commands, enter help at the DOS prompt. I've summarized common commands, below:

• C:: change to a particular drive.
• dir: list files in a directory. Try dir | more to page through long listings.
• tree: show directory tree for current directory.
• cd: change directories.
• For names with spaces, use ~1. For example, to access Program Files on the C drive,
  C:\cd progra~1.
• To move up one directory, use cd ...
• Desktop: I use cd C:\docume~1\dis\desktop. [to add--Shortcuts?]
• type: view contents of a file. For example, type Thing.java.
• edit: create/edit/save a text file.
• copy: copy a file. For example, copy Thing.java ThingCopy.java.
• ren: rename a file. For example, ren ThingOld.java ThingNew.java.
• del: delete a file. For example, del Thing.java.

2.1.2 Mac

We need more info here. For now, see Sun's tutorial.

2.1.3 Unix

Unix windows:

• All hail Unix!
• If you're using Unix, you probably don't need any help :-)
• See here just in case.
• For information on windowing environments in Unix, see http://www.plig.org/xwinman/.

Unix commands:

• http://www.bsd.org/unixcmds.html
• See also Introduction to Unix by David I. Schwartz

2.2 Editing Java Programs

Inside a command window, you can enter all of Java command along with operating systems commands, but your system must be set up to know where the commands reside. The primary Java commands that you need (java ,javac) are located in the directory that contains the JDK programs.

2.2.1 ASCII Text

[need to add this section: how to save from Word, Notepad, Wordpad, using DOS's EDIT, using DOS's TYPE command to view text]

2.2.2 Common Editors

• Microsoft Windows: If you do not wish to install another text editor, use Notepad or Wordpad, but be sure to save your work in ASCII or UNICODE text format. See also the DOS command edit.
• Unix: Use emacs. Enough said. Some people prefer vi. [need to add links]
• Mac: I have no idea what you use at this point.

Note that you can use the text editors that are part of most IDEs.

2.2.3 Other Text Editors

You must create a file in ASCII text format. Some commonly used text editors are as follows:

• Emacs
• SynEdit
• EditPad Lite

2.3 Compiling and Running Java Programs

There are two directory paths (the list of directories from the top to the current directory) that you need to keep track of:

• Java software: Look for a directory name, like j2sdk, jdk, j2se:
  • Microsoft Windows: The Java software commands are somewhere in C:\, like C:\j2sdk1.4.2_03\.
  • Unix: On my system, /usr/j2se.
  • Max: beats me.
  Inside the Java directory, there is a subdirectory called bin. Inside of it, you will see programs called javac and java:
  • javac: This program compiles your Java programs (.java extensions) into class files .class extensions) that contain Java bytecodes. In Microsoft Windows, this program is called javac.exe.
  • java: This program runs your compiled Java classes. In Microsoft Windows, this program is called java.exe.
• Java file: You need to pick a specific place to put your Java files. Generally, you should create a new directory each time you want to work on a new program with multiple classes.

2.4 Path and Classpath Variables

To find your Java files, your operating system needs to "know" which Java files to compile and run. For example, if you do not want to bother setting the Microsoft Windows PATH variable, you might have to enter something like the following command-line to compile a Java program called MyProgram:

C:>\j2sdk1.4.2_yourversion\bin\javac   C:\docume~1\dis\desktop\MyProgram.java

To run a compiled program, you issue this command:

C:>\j2sdk1.4.2_yourversion\bin\java   C:\docume~1\dis\desktop\MyProgram
(don't include the extension .java or .class)

   public class TestArgs {
      public static void main(String[] args) { // could use any name for args!
         for (int i=0; i < args.length; i++)
            System.out.println(args[i]);
         }
      }
   }